The invention relates generally to resource allocation in a wireless code division multiple access communication system. More specifically, the invention relates to assigning uplink and downlink channels in response to access requests of user equipment.
FIG. 1 depicts a wireless spread spectrum Code Division Multiple Access (CDMA) communication system 18. A base station 20 communicates with user equipment (UE) 22-26 in its operating area. In a spread spectrum CDMA system 18, data signals are communicated between UEs 22-26 and the base station 20 over the same spread bandwidth. Each data signal in the shared bandwidth is spread with a unique chip code sequence. Upon reception, using a replica of the chip code sequence, a particular data signal is recovered.
Since signals are distinguished by their chip code sequences (code), separate dedicated communication channels are created using different codes. Signals from the base station 20 to the UEs 22-26 are sent on downlink channels and signals from the UEs 22-26 to the base station 20 are sent on uplink channels. For coherent detection of downlink transmissions by the UEs 22-26, pilot signals are transmitted to all the UEs 22-26 within the base station's operating range. The UEs 22-26 condition their receivers based on the pilot signals to enable data reception.
In many CDMA systems, a common packet channel (CPCH) is used for uplink transmissions. A CPCH is capable of carrying packets of data from different UEs 22-26. Each packet is distinguishable by its code. For detection by the base station 20, the packets have a preamble which also distinguishes it from other packets. The CPCH is typically used to carry infrequently communicated data at high rates.
FIG. 2 depicts a CPCH access scheme 28. The CPCH access scheme 28 is time divided into intervals having time slots 30-34, such as 8 time slots proposed for the Third Generation Mobile Telecommunications System (IMT-2000). A group of predetermined signatures 36-40 are assigned to the time slots 30-34 to allow more than one UE 22-26 to use the same time slot 30-34. A particular signature used within a particular time slot is referred to as an access opportunity 66-82. For instance, for each of the 8 time slots in the proposal for IMT-2000, one out of 16 signatures is available to be chosen, resulting in 128 access opportunities. Each signature 36-40 is preassigned a virtual channel. A virtual channel uniquely defines operating parameters for both the uplink and downlink, i.e., an uplink spreading factor and a unique code for the downlink.
Broadcast from the base station 20 to each UE 22-26 is the availability of each virtual channel over an acknowledge indication channel (AICH). The UE 22-26 monitors the AICH to determine the availability of each virtual channel. Based on the operating parameters required by the UE 22-26 and the availability of the virtual channels, the UE determines the access opportunity to select. Upon identifying a particular access opportunity, the base station 20 sends out an acknowledgment message (ACK) if the corresponding downlink channel is still available. In the proposal for IMT-2000, the ACK simply repeats the signatures 36-40 associated with the access attempt. If the downlink channel is not available, a negative acknowledge (NAK) is sent.
After receiving a corresponding acknowledgment, the UEs 22-26 determine the proper code to recover communications on the downlink channel based on the access opportunity 66-82 used to send the UEs' packet. Either stored in the UEs 22-26 or transmitted on a Base Station's Broadcast Channel is a list of the code assigned to each access opportunity 66-82. This scheme severely increases the packet collision probability and therefore the packet delay which is undesirable.
In some situations, monitoring the AICH is not desirable. At a particular moment, some UEs 22-26 will be operating in a “sleep” mode. In the “sleep” mode, the UE 22-26 only runs when there is a need to send data. Monitoring the AICH during “sleep” mode will both reduce battery life and introduce a delay in the transmission of the first packet. Additionally, when a UE 22-26 borders between two base station's operating areas, monitoring multiple AICHs further exacerbates these drawbacks.
Monitoring creates other problems. It further complicates the UE's receiving circuitry, making the UE 22-26 more expensive.
Monitoring results in a suboptimum use of the CPCH. AICH monitoring provides information when a channel becomes busy. The time at which the channel becomes free is deduced on a worst case maximum packet length. If a packet is not maximum length, the channel will be idle while the UEs 22-26 are waiting to transmit. On the other hand, if monitoring is not performed in such a system, channel availability information is unavailable. The UE 22-26 may randomly choose a busy virtual channel increasing the packet delay by causing a collision. Accordingly, it would be desirable to allow the UEs 22-26 to wait a period shorter than the maximum packet length and provide for some other collision reducing mechanism.
One technique to reduce the possibility of collision is to raise the number of codes, for instance to 128 different codes. In the proposal for IMT-2000 the 128 sequences represent approximately half of the sequences available at the base station 20. Accordingly, this solution is undesirable. Additionally, since monitoring the AICH complicates the UE receiver circuitry increasing its cost, it is undesirable. Accordingly, an alternate approach to assign virtual channels is desirable.